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Plant biodiversity is an important indicator at various scales. At the community and larger scales, the vegetative structural diversity provides important regulatory functions in agricultural landscapes (Newton et al., 2008). The presence of trees and shrubs as boundary plantings and hedges with multiple plant species provide barriers to runoff (for increased capture of water and soil resources), as well as providing important habitat for beneficial insects that often require a high boundary to field ratio (Andow, 1991). Mosaics of different plant life forms across an area are also associated with other important regulatory services; however, metrics at these larger scales are beyond the scope of this manual.
At the farm and field scale, the plant biodiversity indicator provides information about the crop and non-crop populations. Diversification of crops grown, as well as associated plants, involves choices made by a household at the farm level and at the field level. For a given field, crops can be grown in mixed intercropped systems or as sole crops. Non-crop species are sometimes considered weeds and are possibly removed, but often several species are left in the field along with the crops. These non-crop species are also important and should be noted.
The diversity of a landscape is known as gamma diversity, which has two components: alpha diversity and beta diversity. Alpha diversity is simply the number of species in a field type, i.e., the species richness. Beta diversity can be used to compare the diversity of different types of fields and is a useful measure of the evenness of the diversity across an area.
How to operationalize the metric
Method of data collection and data needed to compute the method:
We recommend using transects (2m by 50m) across the farm landscape for characterizing the tree diversity. We recommend two to three transects per hectare. The species and number of individuals (as well as tree height and diameter at breast height = 1.3m) is recorded. The landscape herbaceous diversity (e.g., crops, weeds, invasive species, rare species) can also be assessed at the landscape level by placing 1m2 quadrats at the 0m, 25m, and 50m positions along the transect. For more details on the quadrat method, see the Crop Residue Productivity Indicator – method 1.
The quadrats used for estimating vegetative cover can be used for estimating field level (alpha) diversity and farm level (beta) diversity. For a particular type of field, maize fields for instance, place five to ten quadrats per field and record the species present. Take the average for the number of quadrats. The species can also be categorized as e.g., crop, weed, invasive, legume.
A major challenge is finding people with taxonomic identification skills to quantify the species-level diversity. It is possible to compare the diversity of functional groups even when identifying to species level is not feasible.
Simple characterization of agricultural diversity can be done from survey data if the data includes the number of crops per household, or the proportion of households or fields with sole crops vs. intercrops, and different crop types (e.g., legumes, cereals, tubers, cash crops, perennials vs annual crops).
At the landscape level, a simple proxy for plant biodiversity is the percent of natural habitat in the landscape. This could be estimated from satellite images or from transects through the landscape
Unit of analysis:
Diversity of a field, farm, or landscape can be compared using a range of calculations. At the most basic level, the incidence of a group of interest may be valuable (i.e., the percent of all trees that are native species).
Beta diversity is often calculated as the number of species unique to the two fields being compared. The larger the number, the greater the differences between pairs of fields and the greater the beta diversity. For a more thorough analysis of different concepts referred to as “beta diversity” and how to calculate, them see Tuomisto (2010).
At the landscape level, gamma diversity is the number of species found along the total number of transects.
Often a diversity index is calculated for comparison of one value representing a balance of alpha, beta, and gamma diversity. There are two main indices of diversity: the Simpson index (which is more sensitive to changes in richness) and the Shannon index (which is a better measure of dominance in terms of abundance). Choosing between the two depends on the goals and context.
Through government records or project survey data, these metrics can be calculated by recording the total number of crops per field per season and the total crop area cultivated. From this data, the proportion of area under legumes, cereals, and tubers can be determined, as well as the proportion of area under sole crop or intercrop, defined as two or more crops per field, per season (e.g., Shaxson and Tauer, 1992).
Limitations regarding estimating and interpreting:
Diversity calculations do not reflect the value or function of specific groups or species, so this indicator may best be used in conjunction with other indicators, such as nutrient partial balance, vegetative cover, or pest levels.